Recently, non-volatile semiconductor memory devices capable of retaining data even when the power is turned off have been widely used. In a typical non-volatile flash memory device, a transistor which constitutes a memory cell has a floating gate or an insulating film which is called a charge storage layer. Data is stored by accumulating charges in the charge storage layer. The flash memory with the insulating film as the charge storage layer has a SONOS (Silicon Oxide Nitride Oxide Silicon) structure where the charge is accumulated in the charge storage layer inside the ONO (oxide film/nitride film/oxide film) film. A SONOS type flash memory with a virtual ground type memory cell that exchanges the source and drain to realize symmetrical operations has been disclosed. The use of this type of flash memory allows the storage of 2 bits of data in a memory cell.
Japanese Unexamined Patent Application Publication No. 2005-116964 (Document 1) discloses a non-volatile memory having a charge storage layer formed on a side surface of a trench, and an impurity region formed on an upper surface of a semiconductor substrate between trenches. Japanese Unexamined Patent Application No. 2005-136426 (Document 2) discloses a non-volatile memory having a charge storage layer on a side surface of a trench, and an impurity region formed on a bottom surface of the trench and on an upper surface of a semiconductor substrate between the trenches.
The non-volatile memory as disclosed in Documents 1 and 2 has the charge storage layer formed on the side surface of the trench, and accordingly, the resultant memory cell may be miniaturized. However, in the non-volatile memory disclosed in Document 1, the impurity region as the bit line has its bottom surface in contact with the channel region. Therefore, the electric field between the channel region and the bit line is small, failing to improve writing and erasing features.
The non-volatile memory as disclosed in Document 2 allows the charge storage layer formed on one side surface of the trench to store 2-bit data. This may result in interference in the charge accumulated in the charge storage layer.